1. Field of the Invention
The field of the present invention is optical analysis systems, and more particularly, optical analysis systems that utilize light at specific wavelengths to optically analyze the properties of gases, liquids, or solids.
2. Background
Different optical analysis techniques are currently in use to analyze the properties of molecules that are components of gases, liquids, or solids. One of the more common techniques, used frequently in the analysis of gases, is absorption spectroscopy, whereby light having a wavelength that corresponds to an absorption feature of a particular molecule is directed through a gas sample. The power of the emerging light is measured, compared against the power of the light incident at the sample, and used to determine whether the particular molecule is present and if it is present, its concentration. Other optical analysis techniques that utilize optics to analyze the characteristics of a molecular component are, for example, photoacoustic spectroscopy, fluorescence spectroscopy, cavity ring-down spectroscopy, fiber interferometry, evanescent wave spectroscopy, and scattering spectroscopy. These other techniques may be used to determine properties such as the presence and size of the particular molecule, concentration, temperature, etc.
The absorption spectrum of any molecule must be considered to determine which of the wavelengths it absorbs will yield the best results of any given optical analysis. For example, carbon monoxide has absorption bands in the near-infrared and infrared spectrum centered at wavelengths of approximately 1.56 μm, 2.35 μm, and 4.65 μm. The absorption line strengths of carbon monoxide, however, are not uniform within a band, nor are they uniform across these three different bands. For example, the strongest absorption transition at 1.56 μm is approximately 125 times weaker than the strongest absorption transition at 2.35 μm, and approximately 20,000 times weaker than the strongest absorption transition at 4.65 μm. The absorption spectra of other molecules, such as, for example, carbon dioxide and nitric oxide, show similar trends in absorption strength, with absorption strengths being much lower at the shorter wavelengths in the near-infrared spectrum than at the longer wavelengths in the infrared spectrum or in some instances in the UV spectrum.
Absorption spectroscopy benefits tremendously by utilizing a wavelength that overlaps with a high absorption line strength for the species of interest because the sensitivity of absorption spectroscopy measurements is directly proportional to the absorption line strength and the path length of the radiation through the sample being analyzed. Therefore, an absorption spectrometry analysis of carbon monoxide using the longer wavelength can enhance the sensitivity of the measurements by a factor of 20,000 over measurements performed using shorter wavelengths. The difference in absorption line strength for many molecules may vary by a factor of hundreds to tens thousands of times between the shorter and longer wavelengths in the near-infrared and infrared spectrum, with the longer wavelengths generally yielding greater sensitivity in absorption measurements.
Due to the potential for improved sensitivity during the absorption spectroscopy measurement, strategies that have been developed thus far tend to take advantage of the stronger absorption features in the infrared and UV spectrum. However, lasers and other associated equipment that operate at these wavelengths are bulky and expensive. Therefore, the strategies tend to focus not only on increasing sensitivity, but also on portability and affordability.
The present state of the art teaches that the combination of the following three strategies yields the highest sensitivity increase while also enabling portable and affordable absorption spectroscopy. First, because lasers producing near-infrared radiation are readily available and economical, techniques such as non-linear frequency conversion are often used to convert near-infrared radiation into mid-infrared or UV radiation in order to take advantage of stronger absorption features. In addition, because the conversion process is highly inefficient at low values of near infrared radiation power and it results in an extreme loss of power at the converted frequency, fiber amplifiers may be employed in conjunction with the non-linear frequency conversion process. The fiber amplifiers increase the radiation power available to the non-linear conversion process, thereby partially overcoming the inefficiencies of the conversion process. Second, because the detection sensitivity of absorption spectroscopy is directly proportional to path length in the sample, path lengths are sometimes increased through the implementation of multi-pass optical arrangements, including multi-pass cells. Third, sophisticated techniques such as frequency modulation, auto-balancing, etc., may be employed to increase the signal to noise ratio, thereby increasing the overall detection sensitivity.
The above strategy of generating infrared or UV radiation from near-infrared sources, however, does not provide similar advantages for all molecules because not all molecules have absorption spectrum features similar to that of carbon monoxide. Some molecules, such as ammonia and methane, have absorption bands that increase in magnitude comparatively little from the near-infrared to the mid-infrared spectrum. Ammonia has several near-infrared spectral absorption bands at wavelengths of approximately 1.5 μm, 1.65 μm, 2.0 μm, 2.3 μm, and 3.0 μm, with the strongest absorption transition at 3.0 μm being only approximately 8–10 times stronger than the strongest absorption transition at 1.5 μm. Similarly, methane has spectral absorption bands at wavelengths of approximately 1.65 μm and 3.3 μm, with the strongest absorption transition at 3.3 μm being approximately 75 times stronger than the strongest absorption transition at 1.65 μm. Therefore, the advantages gained through the use of mid-infrared radiation to analyze molecules such as carbon monoxide are not as attractive when analyzing molecules such as ammonia and methane.
A second optical analysis technique, photoacoustic spectroscopy, is recognized as being a very sensitive technique. Photoacoustic spectroscopy, however, has also traditionally been implemented with the longer infrared wavelengths because stronger absorption features are typically found in that spectrum and because of the high power lasers available at those wavelengths. For photoacoustic spectroscopy, the available sensitivity is directly proportional to the available laser power. As with absorption spectroscopy, it is desirable to take advantage of commercially available near-infrared lasers to make photoacoustic spectroscopy more affordable and portable, and as a result, previous studies have used near-infrared lasers to generate infrared radiation corresponding to the desired absorption feature using the aforementioned non-linear frequency conversion techniques.
The problem associated with this approach, however, is that photoacoustic sensors would actually lose sensitivity because of the inefficiencies of non-linear frequency conversion, even if a fiber-amplifier were employed to counteract these inefficiencies. Therefore, other techniques have been developed to increase the sensitivity of photoacoustic sensors using near-infrared sources, such as the ones reported in the study by M. Feher et al., Applied Optics, 33(9): 1655 (1994). In that study, a diode laser operating in the near-infrared spectrum was used to create a simple, inexpensive, and portable photoacoustic spectrometer to perform an analysis of ammonia. In order to compensate for ammonia's low absorption coefficients near 1532 nm and increase the sensitivity of the analysis, the radiation was frequency modulated and a sophisticated resonant acoustic gas cell was employed. These techniques enhanced the signal and minimized the effects of noise during the analysis. The sophisticated photoacoustic cell and the frequency modulated radiation were credited with increasing the sensitivity of the absorption measurements by two orders of magnitude. Achieving such sensitivity increases without the need to employ a sophisticated photoacoustic cell, however, is desirable.
Improved systems and methods are therefore needed to enhance the sensitivity of optical analyses performed using near-infrared radiation. Such systems and methods should not only have sufficient sensitivity, but also improved simplicity.